1. Field of the Invention
This invention relates to a photoelectric smoke sensor which detects light scattered by smoke entering a smoke sensing chamber and transmits a fire signal to a central signal station. More particularly, this invention relates to a photoelectric smoke sensor of this type which is capable of ensuring high reliability, reducing current consumption and lowering a manufacturing cost.
2. Description of the Prior Art
As a conventional photoelectric smoke sensor of the type as specified above, there has been practically used a photoelectric smoke sensor as illustrated in a block diagram of FIG. 1.
In FIG. 1, numeral 1 designates a diode bridge circuit which is provided to obtain an output of a desired polarity irrespective of a change in the connection polarity of power and signal lines 1.sub.1, 1.sub.2 leading to a central signal station. Following the diode bridge circuit 1, there are provided a switching circuit 2 including a thyristor which short-circuits between the power and signal lines 1.sub.1, 1.sub.2 to transmit a fire signal upon detection of a fire, a constant voltage circuit 3 having a current limiting function, an oscillator circuit 4 including a pulse control circuit, a light emitting diode 5 which is intermittently driven in response to a pulse signal from the oscillator circuit, a photo diode 7 which is reversely biased and conducts upon receipt of light scattered by smoke entering a smoke sensing section 6, a comparator circuit 8 which generates an output when a photo voltage obtained upon conducting of the photo diode 7, and a storage circuit 9 which outputs a fire detection signal to energize the switching circuit 2 when two successive outputs are obtained from the comparator circuit 8.
The circuit arrangement as described above has now been standard in the photoelectric smoke sensor. In this circuit arrangement, to reduce a current consumption, not only the light emitting diode 5 is intermittently driven by pulses, but the comparator circuit 8 is intermittently supplied with power from the oscillator circuit 4 in synchronism with the driving of the light emitting diode 5 to operate the comparator circuit 8 only during a period when pulsed light is output. CMOS is used as devices in the circuits to curtail the entire current consumption of the smoke sensor.
In this connection, it is to be noted that the current most consumed in the circuit arrangement of FIG. 1 is a current used to drive the light emitting diode 5 and it reaches 50% of the entire current consumption.
Therefore, it is most effective to reduce a driving current for the light emitting diode 5 to curtail the entire current consumption. However, if the driving current is reduced, the scattered light incident on the photo diode 7 from the smoke detecting section 6 is also reduced and the photo voltage is lowered.
To solve this problem, as the comparator circuit 8, there has been used a comparator circuit as illustrated in FIG. 2 in which a load resistor R.sub.o of several-hundred kilo-ohms is connected in series with the photo diode 7 which is reversely biased with reference to a power source, and a voltage developed across the load resistor R.sub.o by a photo current which flows when the photo diode 7 detects the light scattered by smoke is amplified by an amplifier 11 including an operational amplifier or a transistor amplifier circuit having a gain as high as 500 to 1,000 times to turn on a transistor when the amplification output exceeds about 0.6 V of a base-emitter voltage of the transistor Tr; a comparator circuit as illustrated in FIG. 3 in which a load resistor R.sub.o of several-hundred kilo-ohms is connected in parallel with the photo diode 7, the photo voltage obtained upon detection of the scattered light by smoke is detected in the form of a voltage developed across the load resistor R.sub.o, and amplified by an amplifier 11 which is comprised of an operational amplifier or a transistor amplification circuit having a gain as high as 500 to 1,000 times to turn on a transistor Tr when the amplification output exceeds about 0.6 V of base-emitter voltage of the transistor, or a comparator circuit as illustrated in FIG. 4 in which a comparator 12 which compares an output of the amplifier 11 with a reference voltage Vr is employed.
Alternatively, as disclosed in U.S. Pat. No. 4,186,390, a photo diode is connected between an inverting terminal and a non-inverting terminal of an operational amplifier to amplify, with a high gain, a photo current obtained by short-circuiting therebetween, a transistor circuit is provided to decide whether the output of the operational amplifier reaches a level corresponding to a predetermined smoke density, and an alarm circuit is actuated through a logical circuit comprised of flip-flops.
In the arrangements as shown in FIGS. 2 and 3 and as disclosed in U.S. Pat. No. 4,186,390, a low-cost, two-power source operational amplifier or a transistor amplification circuit including two or three transistors is employed, and to reduce the current consumption by the amplifier, a micropower type two-power source operational amplifier is used in case the operational amplifier is employed and transistors having a high d.c. amplification are Darlington connected and a resistance at the collector or emitter side of the transistor is high to reduce a collecter current at a normal condition in case the transistor amplification circuit is employed.
Or, a common operational amplifier, i.e., an operational amplifier whose current consumption is several milli-amperes may be employed. In this case, to reduce current consumption by the amplifier, a power source is connected to the operational amplifier about several milli-seconds before the driving of the light emitting diode so that the light emitting diode is driven after the operation of the operational amplifier becomes stable, and a power source is disconnected when the driving of the light emitting diode is finished. This idea is disclosed, for example, in U.S. Pat. No. 4,198,627.
With these special arrangement for curtailing the current consumption, the conventional smoke sensor has successfully attained reduction of the average current consumption of the entire system at a normal supervisory condition (a condition where no fire signal is generated) to about 100.mu.A. The specifications of the current consumption are as follows:
(a) constant voltage circuit 3: about 2 to 5.mu.A PA0 (b) driving current of light emitting diode 5: about 40 to 60.mu.A PA0 (c) oscillator circuit 4: about 5 to 10.mu.A PA0 (d) amplifier 11 of comparator circuit 8: about 15.mu.A PA0 (e) storage circuit 9: about 5 to 10.mu.A PA0 (f) leakage current of device: about 5 to 10.mu.A
However, in the case of a system as illustrated in FIG. 2 wherein a photo voltage of several milli-volts is amplified by the amplifier, the comparator circuit 8 generates an inverting output and causes an erroneous operation by a noise as small as 1 mV which is occasionally produced by electromagnetic induction or electrostatic induction. In the case of a system wherein the two-power source operational amplifier is employed, a power voltage in divided by a zener diode or a dividing resistor to obtain a middle point potential. To suppress a current consumption by the zener diode or the dividing resistor, they should be of high impedance and the potential is liable to be fluctuated by noises, possibly causing an erroneous operation.
By this reason, in the conventional smoke sensors, the entire circuit is encased in a shield case 10 as shown by a broken line in FIG. 1 to prevent an erroneous operation by an external noise.
However, even if the circuit is fully shielded by the shield case 10, erroneous operations cannot always be prevented and they will occasionally be caused by an induction noise superimposed in the power and signal lines 1.sub.1, 1.sub.2 because the circuit is connected to the central signal station via the power and signal lines 1.sub.1, 1.sub.2. In addition, a shield case which has a sufficient shielding effect is too expensive. Thus, there has not been provided yet a smoke sensor which can satisfy all the requirements of high reliability, low current consumption and low manufacturing cost.